Responses of Arbuscular Mycorrhizal Fungi to Multiple Global Change Drivers in Coastal Sage Scrub

While the importance of animal and plant responses to global change is generally appreciated, comparatively little is known about fungal community responses to shifting environmental conditions. Our study evaluated the role of aridity, atmospheric nitrogen deposition and annual grass invasion in driving responses of the fungal community in roots, focusing on the arbuscular mycorrhizal fungi (AMF). Previous work has found that AMF with high root colonization (rhizophilic AMF) reduce root pathogen infection and respond positively to nitrogen deposition and annual grass invasion. Other work has found that AMF with high soil colonization (edaphophilic AMF) are more important for nutrient uptake and generally respond negatively to additional nitrogen, drought and association with grasses. We ask: do guilds of AMF respond differently to multiple drivers of global change and does this impact community biomass allocation and interactions between fungi?We sampled roots and soils from native shrubs and invasive grasses in coastal sage scrub within a long-term experiment manipulating nitrogen and precipitation. We extracted and counted spore and hyphal densities from soils, percent root length colonized by AM and non-AM fungi, and measurements of soil chemistry (Total N,C,P, NH4,NO3 and pH). Additionally, we estimated the composition of fungi in plant roots using two rRNA loci: SSU with AMF specific primers and ITS2 with universal fungal primers. We evaluated these responses of fungal composition and biomass with generalized linear models.We found that drought reduces the relative abundance of edaphophilic AMF and overall AMF biomass. Nitrogen addition reduces the relative abundance of edaphophilic AMF, and increases AMF allocation to root colonization and spore production while reducing the extent of nutrient foraging soil hyphae. Both drought and nitrogen deposition increased the extent of root colonization by saprotrophic and pathogenic fungi, suggesting that plants become more susceptible to infection by non-mutualistic fungi as water becomes more limiting. Invasive annual grasses hosted less edaphophilic AMF than did native shrubs, while one grass species also hosted a higher abundance of ancestral AMF. Overall, this suggests that with increasing drought, nitrogen deposition and invasion of shrub communities by annual grasses, the abundance of edaphophilic AMF in coastal sage scrub will decline